Computer systems are used for a wide variety of tasks and are usually directed by an operator. It is now common in the computer industry to provide systems which include provision for the insertion/removal of memory cartridges, wherein the cartridges are either read only storage or read/write storage, or both. Examples of such systems include Wenninger et al U.S. Pat. No. 3,971,925; Hamano et al U.S. Pat. No. 4,220,991; Housey, Jr. U.S. Pat. No. 4,385,366; Takahashi et al U.S. Pat. No. 4,419,738; Lou U.S. Pat. No. 4,454,591 and Daniels, Jr. U.S. Pat. No. 4,481,587.
Wenninger et al for example provides an Adaptable Programmed Calculator with a provision for plug in memory modules. These plug in ROMs can contain programs which can be executed by the user from the keyboard. Hamano deals with electronic cash registers which accept plug in random access (read/write) cartridges. Housey, Jr. describes a programmable device in which memory modules are selectively connected (such as by a plug in cartridge) read only memories to personalize the device. Takahashi describes a system in which a plug in read/write memory is used to transfer data to a dedicated read/write memory in an electronic cash register. Lou describes an interface system for bus line control in an electronic calculator which employs, in addition to fixed memory modules, plug in memory modules.
Finally, Daniels, Jr. describes an apparatus for providing interchangeable keyboard functions, which apparatus includes insertable memory modules.
In some systems in which read only or read/write memory can be inserted/removed, operating instructions require that the insertion/removal be accomplished when the system is powered down, an example of such a system is the last-mentioned Daniels, Jr. patent. He describes (see col. 8, lines 53 et seq) that after ROM insertion is effected, the "user then turns the scale 112 on by supplying power thereto". Video games and the PCjr are other examples of systems which include memory cartridges which can be inserted or removed, but in which the insertion/removal is effected when the system is powered down. In many cases, however, while the insertion/removal of the memory cartridge effects a reset operation, data is to be retained across this reset; that is, data stored prior to the reset is to be retained so that it can be used after the reset. In other systems, or other applications of the same systems, there is a desire to maintain the system powered up even though a memory cartridge may be inserted/removed. We have found that the insertion/removal of a cartridge can, in response to the electrical disturbances caused by the insertion/removal, produce unwanted changes and/or erroneous operations. The insertion/removal of an electrical component will, by reason of contact bounce and similar phenomena, produce undesired changes in electrical voltages on the conductors connected thereto. These undesired voltage changes can be catastrophic. For example, the undesired voltage changes may alter a word on a data or address bus which can result in:
execution of an incorrect instruction; or
access of an incorrect memory address (for either a read or a write operation).
The consequences of noise on a data or address bus has been recognized in the prior art, see Hartlipp et al U.S. Pat. No. 3,548,177; Shutler U.S. Pat. No. 3,548,176 and Carnevale U.S. Pat. No. 3,548,178. In these patents, a noise detector is provided in the computer system which, in the presence of noise, initiates some action to inhibit or minimize the consequences of the noise. These references are not related to memory cartridges which can be inserted/removed, require the addition of noise detector hardware, and may slow down machine execution